Lua filters: iterate over AST element fields when using `pairs`

This makes it possible to iterate over all field names of an AST element
by using a generic `for` loop with `pairs`:

    for field_name, field_content in pairs(element) do
      …
    end

Raw table fields of AST elements should be considered an implementation
detail and might change in the future. Accessing element properties
should always happen through the fields listed in the Lua filter docs.

Note that the iterator currently excludes the `t`/`tag` field.

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